Road signs are used to inform drivers of road conditions and regulations as they relate to the area in which the sign is located. Many such signs are currently made using a “retro-reflective” material, that is, a material which reflects light directly back in the direction from which it came. This property greatly increases the visibility of such signs at night by allowing light from the headlights of passing vehicles to be reflected straight back towards the driver. However, such signs can only be seen at night when they fall within the beam of a vehicle's headlights.
Earlier awareness of the road conditions and regulations gives a driver additional time to respond to such information, reducing the likelihood of accidents. It is therefore sometimes desirable to provide illuminated signs which can be seen at night even when they are not illuminated by headlights. Generally, such illumination is achieved by lamps arranged above or below the sign and directed onto the sign from in front. An alternative is to illuminate the information face of the sign from behind by using a semi-transparent face with a light arranged therebehind, shining through the face. Such signs are familiar for example from traffic bollards at junctions, which have a “light box” containing a light source, light being emitted through four faces. Such backlit signs generally have no retro-reflective properties and therefore rely fully on the backlighting for visibility.
Furthermore, signs illuminated by lamps, whether from in front or from behind, require a significant power input, which is expensive. Furthermore, in remote regions, it may be that there is no nearby source of mains electricity. Such signs are also costly to maintain and install.
One method of reducing the power required for such signs is disclosed in WO00/48166. A sign panel is illuminated from behind by a flat panel of electroluminescent material. The front of the sign comprises a retro-reflective lens.
The sign also includes a photodetector for measuring ambient light, and the electroluminescent material is only activated so as to emit light when the ambient light falls below a predetermined level—i.e. during the night.
This system works well, since electroluminescent materials require a much lower power input than conventional lamps to produce light of sufficient brightness. Since the sign is only on during the night, the long term power consumption is greatly reduced.
However, the addition of photodetectors increases the complexity of the signs. Since road signs have to be produced in large numbers, complex components are undesirable.
Furthermore, electroluminescent materials have a finite lifetime. For example, a typical material may initially generate light at 34.6 cd m−2 when supplied with alternating current at 100V and 400 Hz. After 3,500 hours of continuous use, the light emitted under these conditions will have fallen to 30% of its initial brightness. One way to increase the lifetime of an electroluminescent light source is to decrease the voltage and frequency of the electricity supplied: for example, the material described above, if supplied at 100V ac and 60 Hz reduces to 30% brightness after 20,000 hours. However, the initial brightness is also reduced so the increased lifetime comes at a cost.
There is therefore a need for a sign which combines the added visibility of an actively illuminated sign with the safety of a retro-reflective sign. There is a further need for illuminated signs with a low power requirement. There is a yet further need for such signs which can be simply manufactured. There is a yet further need for signs with an increased working lifetime.
In accordance with a first aspect of the present invention there is provided an illuminated sign comprising:                a sign panel; and        at least two elements of electroluminescent material, each element being arranged so that it can emit light so as to illuminate the sign panel, each element being selectively actuatable independently of the or each other element.        
This arrangement allows the possibility that the elements can be used one at a time to illuminate the sign panel, for example two elements could be used on alternate nights. Since each element is being used less than would be the case if there was only a single element, they degrade less quickly and the lifetime of the sign is greatly increased.
Electroluminescent material is cheap to manufacture and install, and furthermore requires a much lower power input than conventional lamps. Electroluminescent materials can be formed into films, enabling the light source to be arranged as a thin film directly behind the sign panel. The use of electroluminescent film as the light source allows the source to be designed in such a way that only part of the area of the sign emits light.
Electroluminescent material also has the property that its capacitance varies with the amount of light falling on the material. By measuring the capacitance of one of the elements, it can be used as a photodetector to measure the brightness of external light falling on the sign.
Preferably, therefore, each element can be used either as a photodetector to measure the amount of external light falling on the sign, or as a photo-emitter to emit light to illuminate the sign panel. This removes the need for separate photodetectors, thereby reducing the number of complex components in the system.
One method for measuring the capacitance of an element involves the application of a constant voltage to that element. However, in order to activate an element to cause it to emit light, an alternating voltage must be applied. It is therefore not practical to use an element as a photo-emitter and photodetector at the same time.
Accordingly, when one element acts as a photo-emitter, the or each other element preferably acts as a photodetector.
Preferably the sign is arranged so that none of the elements emits light when the brightness of external light falling on the sign is above a predetermined level. This level is preferably defined such that none of the elements emits light during the day. Thus the sign will only be illuminated during the night.
Preferably, each element comprises segments interlocking with corresponding segments in the or each other element so that the segments form a spatially alternating array. This ensures that the whole sign panel will be illuminated when only one of the elements emits light.
As discussed above, the brightness of light emitted by electroluminescent material will reduce with the use of that material. The brightness of emitted light is also affected in the short term by the temperature of the material, which can vary significantly even through 24 hours. In order to compensate for this, the sign may also comprise a control system for regulating the power supplied to the elements. This system is preferably arranged to ensure that the brightness of light emitted by an element remains substantially constant over time, both through one 24 hour cycle and for the lifetime of the sign.
In order to compensate for changes within a 24 hour cycle, the control system may include power measurement means for measuring the power drawn by an element when it is acting as a photo-emitter, and a feedback loop to determine the temperature based on the power drawn by the element. Alternatively an independent temperature measurement means may be employed.
In the long term, the power supplied to each element may be regulated to compensate for degradation so that the brightness of light emitted by the elements remains substantially constant over the lifetime of the sign.
The control system may also determine whether or not each element switches between use as a photodetector and photo-emitter on a regular basis (for example once per day). If not, it is likely that the sign has developed a fault. If this is the case, all the elements are switched so that none of them emits any light. It will then be obvious in a routine check that the sign is faulty, and the fault can be rectified by replacing the control electronics.
Another way to increase the overall lifetime of the sign is to separate the light source into an array of elements or pixels activatable independently of each other. Then, for example, every other pixel is activated initially. The remaining pixels can be kept as emergency backup in case there is a failure with the primary pixels, or to use after the primary pixels have outlived their useful life.
In accordance with a second aspect of the present invention there is provided an illuminated sign, comprising a sign panel and two arrays of light sources arranged to illuminate said sign panel, each array being selectively activatable independently of the or each other array, the light sources of each array being spatially alternated.
One array may be arranged to be activated if the other array fails. Alternatively or in addition, one array may be arranged to be activated after the end of the useful life of the other array.
The sign is preferably arranged so that the arrays are only activatable when external light falling on the sign falls below a predefined level, so that the sign is not illuminated during the day.
The sign may be arranged so that the arrays are selectively activated in sequence. In other words, one of the arrays may be activated one night, and the other array the following night. Alternatively, the arrays may be selectively activated in a random sequence.
Of course, it will be appreciated that there may be more than two arrays of light sources. Three or more arrays may also be employed, operating in sequence, or in a random sequence, or with two arrays alternating and one kept in reserve.
Preferably the arrays of light sources comprise an electroluminescent material, which may be usable to detect the level of external light falling on the sign.
The illuminated sign preferably further comprises a retro-reflective portion. This may be arranged behind the sign panel, although it may form part of the sign panel itself. It will be understood that it is not necessary for the whole of the illuminated sign to be retro-reflective.
The sign preferably comprises detection apparatus for detecting the approach of a vehicle, the sign being arranged so that the light source is activated only if a vehicle is approaching. This can reduce the power consumption even further, so that the sign is only illuminated at night and only when a vehicle is approaching. The apparatus for detecting the approach of a vehicle may take any suitable form, including detecting sound or vibration in the road caused by approaching vehicles,
In accordance with an embodiment of the present invention there is provided an illuminated sign assembly, comprising an illuminated sign as described above and a power source. This enables the illumination of the sign at places where mains electricity is not available.
The power source may be a rechargeable battery, and the sign assembly preferably comprises recharging means for recharging the rechargeable battery. The recharging means may comprise a photovoltaic cell, and/or a device for generating electricity using vibration from passing vehicles.
In accordance with a third aspect of the present invention there is provided a method of illuminating a road sign comprising a sign panel and first and second electroluminescent light sources, the method comprising:                measuring the capacitance of the first light source;        determining the level of external light falling on the sign from the measured capacitance; and        illuminating the sign panel with the second light source if the external light falls below a predefined level.        